Lead alloys having improved corrosion resistance

ABSTRACT

LEAD ALLOYS DISTINGUISHED BY IMPROVED CORROSION RESISTANCE, PARTICULARLY AGAINST SULFURIC ACID, AND IMPROVED CREEP RESISTANCE, SUITABLE ALLOYS OF THIS KING ARE LEAD ALLOYS CONTAINING-BESIDES COMPONENTS FOR REDUCING GRAIN SIZE, ASSISTING THE FORMATION OF PROTECTIVE LAYERS AND INCREASING CREEP RESISTANCE, SUCH AS COPPER, NICKEL, TELLURIUM AND TIN-AT LEAST 0.05% OF PALLADIUM BASED ON THE TOTAL WEIGHT OF THE ALLOY.

May 23, 1972 H. GRAEFEN ETAL LEAD ALLOYS HAVING IMPROVED CORROSION RESISTANCE 2 Sheets-Sheet 1 Filed March 17, 1970 000- OOm 09 INVENTORS m 0 N 13 E0 ..W A n R A K .6 E T B UEM W w B CORROSION RESISTANCE May 23, v1972 H. GRAEFEN EI'AL I LEAD ALLOYS HAVING IMPROVED 2 Sheets-Sheet 2 Filed March 17, 1970 m OE :0 Qmm nm www mm n1 AT T'YS United States Patent Office Patented May 23, 1972 3,664,831 LEAD ALLOYS HAVING IMPROVED CORROSION RESISTANCE Hubert Graefen, Lampertheim, and Dieter Kuron, Mannheim, Germany, assignors to Badische Anilin- & Soda- Fabrik Aktiengesellschaft, Ludwigshafen am Rhine,

Germany Filed Mar. 17, 1970, Sen-No. 20,179

Claims priority, application Germany, Mar.

P 19 14 210.0 Int. Cl. C22c 11/00 US. Cl. 75-166 Claims ABSTRACT OF THE DISCLOSURE This invention relates to lead alloys having improved corrosion resistance, particularly against sulfuric acid, and improved creep resistance.

Due to the generally good corrosion resistance of lead and lead alloys to sulfuric acid, liquids containing sulfuric acid and to sulfate-containing solutions, apparatus and other plant parts made of lead or lead alloys or provided with lead linings are used in industry on a large scale.

The range of possible applications of leads and lead alloys available at present is restricted, from the corrosion point of view, by the dissolution of the lead which increases with increasing concentration and increasing temperature of the sulfuric acid. One such limit for the use of lead has hitherto been set by 45% boiling sulfuric acid for example. The range of applications is also restricted, from the point of view of tenacity, by the relatively poor creep resistance of the previously known leads and lead alloys even at only moderately elevated temperatures.

The corrosion resistance of lead and lead alloys to sulfuric acid is due to the formation of a protective layer of lead sulfate. To enable such a protective layer to be formed, a corrosion process must first take place, which is then reduced to a small corrosion rate by the formation of the protective layer of lead sulfate. On account of the relatively high hydrogen overpotential of lead, however, the formation of a protective layer on pure lead is delayed or completely suppressed. For this reason it is known, for example, to add alloying components such as copper or nickel to the lead, as such components have a low hydrogen overpotential, that is, a higher elfective current density. The corrosion of commercially pure lead (Pb 99.985%) and commercial lead alloys (Pb 99.9%, Cu; hard lead 1.5% Sb) by 50% and 70% boiling sulfuric acid solutions is given in Table 1 in mm./year.

TABLE I.CORROSION BEHAVIOR OF COMMERCIALLY PURE LEAD AND COMMERCIAL LEAD ALLOYS Linear corrosion rate in mm./year in boiling sul- 1 Complete dissolution.

However, not only the magnitude of the hydrogen overpotential of the alloying component is responsible for rapid and effective formation of the protective sulfate layer, but also the size of the cathode area formed, which in turn depends on the solubility and distribution of the alloying component.

Research on the influence of cathodically effective alloying components has shown, surprisingly, that lead alloys have much greater corrosion resistance, particularly against sulfuric acid, and greatly increased creep resistance when they contain, in addition to alloying components for reducing grain size, assisting the formation of protective layers and increasing creep resistance, such as copper, nickel, tellurium and tin, at elast 0.05%, preferably from 0.1 to 0.2%, of palladium based on the total weight of the alloy.

Such small additions of palladium to the commercial lead-copper alloy Pb 99.9 Cu greatly reduce their rates of corrosion in 50% and 70% sulfuric acid, as may be seen from Table 2, line 1. Moreover, we have found that the addition of at least 0.05% (preferably from 0.1 to 0.2%) of palladium to lead alloys already containing from 0.05 to 0.15% of nickel and from 0.1 to 0.15% of tin, lead alloys already containing from 0.05 to 0.15 of nickel and from 0.1 to 0.15% of tin and lead alloys already containing from 0.05 to 0.15% of tellurium and from 0.1 to 0.2% of tin causes a further surprising increase in the corrosion resistance (see Fable 2, lines 2 to 5) and an unexpected improvement in their creep resistance compared with previously known lead alloys. For economic reasons, the concentration of palladium in the lead alloys will be kept as low as possible.

TABLE a-CORROSION BEHAVIOR OF LEAD ALLOYS ACCORDING TO THE INVENTION [The figures in brackets indicate the percentage concentration of the alloying components] Linear corrosion rate in mun/year in boiling sul- Iuric acid having a concentration of FIG. 1 shows the creep curves of commercially pure lead (Pb 99.985), commercial lead-copper alloy (Pb 99.9 Cu), a lead-copper-palladium alloy according to the invention and a lead-copper-tin-palladium alloy according to the invention. The test temperature is 40 C., and the alloys were subjected to loads of 0.6 kg./mm. and 0.4 kg./mm. The Brinell hardness of these alloys, as measured with a ball 2.5 mm. in diameter under a load of 3.1 kg. for a period of 120 seconds is 3.6, 3.6, 4.6 and 6.5 kg./mm. respectively.

The samples of pure lead (Pb 99.985) broke under a load of 0.6 kg./mm. after only about 2 hours and under a load of 0.4 kg./mm. after about hours (rupture indicated by x on the curves). The commercial leadcopper alloy (Pb 99.9 Cu) also broke under a load of 0.6 kg./mm. Samples of the lead-copper-palladium alloy (Pb 99.9 Cu+0.1 Pd) did not break when subjected to a load of 0.6 kg./mm. for 700 hours, although the elongation had at his point exceeded 20%. The lead-coppertin-palladium alloy, however, showed unexpectedly high creep resistance. After almost 1,000 hours under a load of 0.6 kg./mm. this alloy showed an elongation of only about 2%. Under a load of 0.4 kg./mm. the elongation was less than 1%.

In view of these results it appears advisable for the lead alloys of the invention to contain tin in addition to palladium. The addition of antimony, known from the literature as a method of increasing the tenacity of lead alloys, does not result in the same increase in creep resistance and corrosion resistance as the use of tin as alloying component.

FIG. 2 shows the quasistationary anodic current density/potential curves for the two commercial leads (Pb 99.985 and Pb 99.9 Cu) and the two lead alloys of the invention (Pb 99.9 Cu+0.1 Pd and Pb Cu Sn Pd (0.05; 0.12; 0.10)) in boiling 70% sulfuric acid. The current density/potential curves illustrate the two essential effects of the lead alloys of the invention responsible for the increase in corrosion resistance. On the one hand, the steady or corrosion potentials of the lead alloys of the invention have much higher positive values (+500 to +600 mv.), and on the other hand the constant dissolution current density is only about 1X10" amps/cm. as against a value of about 8 l0' for the commercial leads.

The addition of platinum or gold instead of palladium produces an improvement in corrosion resistance and creep resistance of the lead alloys but does not give the unusually good results obtained by adding palladium.

The main advantages of the lead alloys of the invention over the commercially available leads are their exceptionally high corrosion resistance especially to sulfuric acid and their exceptionally good creep resistance.

We claim:

1. Lead alloys having improved corrosion resistance, particularly against sulfuric acid, and improved creep resistance, which contain, in addition to such alloying components as reduce grain size, assist the formation of protective layers and improve creep resistance, said alloying components selected from the group consisting of copper, nickel, tellurium and tin, a small amount of palladium, which is at least 0.5% based on the total weight of the alloy.

References Cited UNITED STATES PATENTS 2,145,513 1/1939 Jones et a1 -166 EX 2,171,180 8/1939 Jones 75\166 EX 2,348,333 5/1944 DEustachio 75166 D 3,147,114 9/1964 Hack et a1. 75166 D FOREIGN PATENTS 414,606 8/1934 Great Britain 75-166 E L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R. 75-166 D, 166 E g ggg UNHED STATES ATET FFHQE mwwm'm Patent: No. 3, 664,851 7 Dated May 23, I 1972 Inventofls) Hubert Gra-efen and Dieter Kuron It is certified that error appears in the above=identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, 1111? iwf vlkast'ffi shoulslflneaieleast, "19

Column 4, line 5, claim 1, "0.5%" should read 0.05%

Signed and sealed this 20th day of November 1973..

(SEAL) Attest:

RENE D. TEGTMEYER Acting Commissioner of Patents EDWARD M.FLETCHER,JR Attesting Officer 

